1. Field of the Invention
This invention is related to a spectrophotometer and in particular, to an improved spectrophotometer.
2. Description of the Prior Art
The following prior art shows spectrophotometers and other color measurement devices:
U.S. Pat. No. 3,245,305 issued Apr. 12, 1966 to Turner is directed to a spectrometric apparatus having two sources of radiation, two monochromators and two detectors which compare the intensities of the two sources of radiation and adjusts the intensity of one of the sources to maintain the intensity of the sources at a constant ratio.
U.S. Pat. No. 3,431,054 issued Mar. 4, 1969 to Doonan, et al. is directed to a scanning monochromator device that has a controlled output of radiant flux over the band of wavelengths to be scanned that is controlled according to an adjustable preset pattern that varies with wavelength.
U.S. Pat. No. 3,506,358 issued Apr. 14, 1980 to Hiroack Baba, et al. is directed to a rapid scanning spectrophotometer of a double beam mode in which a photoelectric detector is provided in each light path and the supply voltage is controlled so that the output of each detector is always at a constant level to provide for variations of intensity of light source used in the spectrophotometer.
U.S. Pat. No. 3,646,331 issued Feb. 29, 1972 to Lord is directed to a method and apparatus for correcting radiation measuring errors in a spectrophotometer. Correction factors for each discrete wavelength are stored in a computer and applied to the output of the apparatus when measurements are made.
U.S. Pat. No. 3,806,258 issued Apr. 23, 1974 to Rich is directed to an absorptiometer in which the lamp source output is maintained at a predetermined level and means are provided to maintain the output at this level while measurements are being made.
U.S. Pat. No. 4,093,991 issued June 6, 1978 to Christie, Jr., et al. is directed to a scanning spectrophotometer-digital data processing system which includes a microcomputer.
U.S. Pat. No. 4,176,957 issued Dec. 4, 1979 to Meada, et al. is directed toward a method for optically analyzing a specimen by an automatic wavelength scanning operation by using two light beams for determining the reflectance of a specimen by using a ratio of the intensities of the reference light and the specimen light at various wavelengths.
Thus prior art spectrophotometers that use a scanning type monochromator to produce an opticalelectrical signal give inaccurate results that vary with wavelength. This variation in signal is due to the emitted spectral distribution of the light source, the wavelength dependent diffraction efficiency of the monochromator and the spectral sensitivity of a photodetector. Other variations in signal level for example are caused by power line voltage variations, thermal expansion of the optical elements, blackening of the source, and when quartz-halogen lamps are used, repetitions of the halogen cycle.
The prior art spectrophotometers are not adequately equipped to compensate for the above variations in the output signals.